1. Field of the Invention:
This invention relates to a method and apparatus for horizontal drilling. More specifically, but not by way of limitation, this invention relates to lateral drilling in oil and gas wells, coal degassification wells, geothermal wells and the like.
2. Description of the Prior Art:
It is generally known that the final portion of a rotary drilling string (the so-called drilling collar) or the like is under compressive loads and torque during drilling, while the upper portion of the drilling string is under tension. As such, the weight of the drill collar below the point of last contact with the borehole wall may be thought of as being divided into two components; one acting along the axis of the collars and the second acting normal to the first, i.e. perpendicular to the borehole.
It is also generally known that, in principle, if the downhole orientation and magnitude of the normal force could be controlled during drilling, the drill bit could essentially be steered to any desired subsurface location or strata. Although theoretically possible and highly desirable, such a process and corresponding drilling equipment to achieve such a goal have not yet been developed. However, various processes and associated equipment have been employed that generally achieve varying degrees of what is recognized in the art as directional drilling. For example, it is common practice in oil and gas well drilling to use a so-called "whipstock" (a sloped plug inserted below the drill bit) to intentionally deflect the drill bit in a desired direction, thus creating a deviation in the direction of drilling.
It is also a common practice in oil and gas well drilling to employ equipment and method for minimizing or eliminating the effect of the force normal to the borehole, such as to maintain the drilling in a vertical line. Thus, various types of drilling collars, stabilizers and the like have been proposed to keep the drilling process proceeding uniformly in one direction. For example, U.S. Pat. Nos. 3,145,785 and 4,319,649 disclose drill collar stabilizers to maintain the drilling in a straight line.
In U.S. Pat. No. 4,220,213 an eccentric member having a heavy, thick walled side and a lighter, thin walled side is placed concentrically about the drill bit collar of a rigid drill string with an offset protection on the outside of the eccentric member positioned usually 90 degrees to the right of the heavy, thick walled portion. In this manner, gravity will cause the heavy, thick walled portion of the eccentric collar to rotate to the underside or low side of a deviating drill string, thus positioning the projection such as to alleviate or compensate for the undesirable "walking" of the drill bit. In other words, an eccentric tubular member rotatably supported on the drill string is used to prevent the drill bit from moving laterally and the resulting normal force continuously restores the hole to vertical.
Various types of drilling collar stabilizers have also been proposed to alter the direction of drilling. For example, U.S. Pat. Nos. 4,305,474 and 4,465,147 disclose stabilizers that create a deflecting force perpendicular to the drill string in order to control and guide the drill bit along a desired course of direction. Also, the use of an eccentric stabilizer has been proposed in U.S. Pat. No. 4,076,084 to drill a directionally oriented hole such as commonly practiced when drilling from an offshore platform or the like.
One particularly difficult type of drilling process to control is the so-called lateral drilling or horizontal drilling. Unlike the concept of directional drilling wherein radii in terms of thousands of feet and deviations up to one to two miles are to be achieved relative to the surface location of the drilling platform or drilling rig, the concept of lateral drilling involves creating a highly curved well bore usually as an offshoot from a pre-drilled well bore. Thus, for example, in U.S. Pat. No. 4,402,551 a whipstock is employed to drill short radius horizontal holes below a vertical cased well bore. It is acknowledged in this patent that current state-of-the-art techniques limit the smallest radius of curvature for surface drilling to 19 feet (i.e., 3 degrees of deflection per foot over 30 feet of linear drilling) to produce a horizontal drainhole at depths greater than 2,000 feet. It is this particular type of highly curved or tight radius drilling of oil and gas wells and the like that the present invention addresses.
In prior attempts to use rotary drilling techniques to achieve a tight radius of curvature (see for example, U.S. Pat. No. 2,687,282) a section of flexible drilling string made up of short segments interconnected by universal joints which in turn terminate in a ball and socket connection to a drill bit collar with stabilizer and rotary drill bit were employed. Although such apparatus could drill a well bore with a tight radius of curvature, the method and apparatus were unacceptable in that the radius of curvature would experience a significant deviation out of the plane of rotation. This in turn would result in a spiral or helical contribution to the curved part of the well bore resulting in an inability to achieve control of either the inclination or horizontal direction of the drainhole or lateral drilled offshoot. The present invention is viewed as being an improvement over the prior art methods and apparatus in that controlled curved drilling is achieved.